Anti-covid vaccines: when are we well protected? – The vaccination campaign in question

  • 1. How long should I wait after the first dose?
  • Pfizer-BioNTech, Moderna and AstraZeneca, all two-dose vaccines available in France show good efficacy after the first injection: the first, which represents more than three quarters of the doses administered in the country, would reduce the risk of developing by 57%. a covid “symptomatic” and 62%, the risk of having a severe form, according to a study published in The New England Journal of Medicine (NEJM), in mid-April.

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    These results were observed between the 14th and the 20th day after the first vaccination. Because you have to be patient: an injection does not mean immediate protection. “The knowledge acquired over decades on the functioning of the immune system and vaccination indicates that it takes between 10 and 15 days for our immune system to trigger a response and produce the first antibodies that will protect us”, explains Sandrine Sarrazin, researcher at the Marseille-Luminy immunology center (CNRS / Inserm / Aix-Marseille University).

  • 2. Is the second injection really useful?
  • The good efficacy observed after the first injection may suggest that this is enough … “No, the second injection is essential for maximum protection”, answers Sandrine Sarrazin. A finding that is confirmed in the study of the NEJM. The protection offered by the Pfizer-BioNTech vaccine climbs to 94% for symptomatic covid and to 92% for severe forms, at least seven days after the second injection.

    “The first injection allows you to recognize the pathogen and therefore to select, in your immune system, the cells which will produce specific antibodies. The second will really allow the system to start up very quickly and in a very powerful way. Two training sessions are better than one, ”summarizes the immunologist.

    Why, then, is the Janssen vaccine only delivered with one dose? “The clinical trial was carried out like this, because we were in an emergency. The laboratory had arguments: technology based on a live adenovirus provides an additional “trick” in stimulating the immune system, compared to RNA vaccines. But the lab is currently testing the principle of a second injection and I would not be surprised if this vaccine offers better protection with a booster, ”says Sandrine Sarrazin.

  • 3. How long will the protection last after the second dose?
  • One year, two years, ten years? Cannot predict it with a mathematical formula. Hindsight is also lacking, according to experts. There are, however, two ways to start evaluating it: “We can measure the level of antibodies in the blood of patients who have done clinical trials, or, in real life, in thousands of people in different countries,” notes Sandrine Sarrazin. Or time will tell: if next winter a new wave breaks out, we can check whether people are still protected. “Experience will show it to us. ”

    Reassuringly, the scientist recalls that “in terms of the theory of immunology, there are not many vaccines that do not confer very long-term immunization. For DT polio, for example, we reboost the immune system every ten or fifteen years, as it ages. There is not really an example of immunization that disappears overnight ”.

  • 4. Will a third injection be necessary?
  • The hypothesis of a recall this winter, to be better protected, seems to be strengthening. “But there is a lot of confusion on this subject,” notes Sandrine Sarrazin. According to the immunologist, this possible third dose “does not aim to maintain a longer efficacy but to provide protection against variants.

    For the moment, the current variants are well controlled by the vaccines we have. But if the virus changes enough for our antibodies to no longer recognize it, then we will have, not a third identical dose for a booster, but a new vaccination with a reformulated vaccine ”.

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